This application is conceived on the premise that in certain diseases, e.g. peripheral artery disease (PAD), traditional therapies are ineffective in the treatment of symptoms and related morbidities. PAD currently impacts over 8 million Americans and will affect many more as our nation ages. The health care costs to treat subjects are significant ($4 billion annually by Medicare), with most patients requiring numerous endovascular and surgical procedures including amputation. Medical therapy has failed to impact the trend in this cost and efficacy curve. This proposal seeks to change this paradigm by recognizing the endothelial dysfunction attributed to PAD prevents proper blood flow recovery (vasodilation and angiogenesis) because the production of nitric oxide through its cognate enzyme nitric oxide synthase is impaired. We have found nitric oxide (NO) precursors in blood and tissues which are stimulated by red light energy to release NO and produce a stable NO bound vasodilator. Furthermore, the release of this autocrine factor dilates arteries/arterioles in the absence of nitric oxide synthase (NOS). The actions of red light are clinically relevant, as we have identified significant elevations in blood flow when the gastrocnemius muscle of healthy subjects and patients with PAD are exposed to red light. The approach in this application will be to assess the impact of these iron and thiol based NO precursors using biochemical and cell culture techniques, optimize light delivery in a murine model of subacute hindlimb ischemia (to mimic clinical PAD), and confirm our findings in human subjects with PAD. We expect the data collected will identify the mechanisms by which NO intracellular NO precursor molecules can be increased, improve endothelial dysfunction and enhance limb perfusion.